For buildings the future is bright - bright green, that is. New research into how stone facades will be altered by changes in the atmosphere suggests that the days of smutty grey and black buildings are gone.
The coming century will see iconic limestone structures like the Empire State Building, the Pentagon, and the gothic cathedrals of Europe and the US turn yellow, reddish-brown, and even green with lichen and moss.
Cities will become more colourful as pollution patterns change and wind-swept rain washes away the black coal soot typical of the 20th century. What's more, legal requirements to use clean fuels are likely to mean lichens and mosses will grow more easily, turning buildings green in parts.
In the 19th and 20th centuries, cities in Europe and the US were dominated by dark grey and black buildings. These were often made of cream-coloured stones like limestones, covered in a black crust of coal soot. According to Peter Brimblecombe and Carlotta Grossi of the University of East Anglia in the UK, the era where atmospheric pollution determined the damage to building materials is over.
The pair recently completed an assessment of how damage to buildings in London has varied over the past 900 years and how it is likely to evolve over the coming century - when soot from coal-burning stoves is unlikely to be a concern, but climate change is.
They used historical climate and pollution data, combined with equations that describe how different climates and different types of pollution affect building materials. For instance, archive tax records show how much fuel was used through the centuries, which can be used to estimate historical pollution levels.
With estimates of how much black carbon soot deposited on buildings at different times, the researchers can calculate the reflectivity of buildings, which indicates how black they were. It turns out that for most of the 900 years, buildings were clean.
"It seems the past two centuries [of blackened buildings] were a bizarre anomaly," says Brimblecombe. "In a sense, we are now back in medieval England."
A study on a limestone building known as the Cathedral of Learning at the University of Pittsburgh, using pictures taken throughout its 20th century construction, shows that the cream-coloured façade was black even before the structure was completed in the 1930s.
In subsequent decades, however, air-quality regulations meant black crusts of soot became less of a problem, and parts of the building that get most soaked with rain have been naturally cleaned.
Climate models predict that temperate regions are likely to receive more rain with climate change, meaning cities like London will become lighter. But the natural removal of black and grey deposits is likely to reveal other changes in the stone, says Grossi.
Already, she and Brimblecombe have shown that switching away from coal fuels has made the Tower of London slightly yellow and reddish-brown hues. This is the result of the oxidisation of organic compounds in diesel and petrol fumes. On this and other buildings, the hues may have been there for some time, but would have been hidden by the black carbon crusts.
Cliff Davidson of Carnegie Mellon University, who carried out the Cathedral of Learning study, says these changes in hue could be boosted by warming climates. "This will cause a greater fraction of [organic fuel compounds] to become gases rather than particles," he says. "Gases may deposit more quickly on buildings."
Fossil fuels are changing too. Crucially, car and truck fuels now emit less sulphates than they used to. Sulphates suppress the growth of lichens and mosses, so cleaner fuels are likely to mean greener buildings.
According to Grossi, buildings will not be homogenously green, or yellow, or reddish-brown, but different colours in different places. Mosses and lichen, for instance, prefer humid environments and will probably colonise cracks and corners.
Whether or not we notice the subtle changes remains to be seen. Grossi has carried out surveys on visitors to a cathedral in Norwich in the UK. The results suggest that we are likely to notice if a building becomes darker or lighter, but are more oblivious to hue changes.
With lighter, cleaner buildings overall, though, she believes we may become more sensitive to the yellows, browns, and greens.